I would LOVE to visit a lot of times. Midevil, during both world wars in various areas to see how the world reacted to stuff than, ancient egypt, walk the streets of Rome during the height of its reign of Cesar, visit Washington DC during the creation of our country, etc. Basically would love to witness the significant events in history or just hang around a couple days during some of the greatest dynasties in history to see how the people spent their days and how the cities looked.
But to live... I wouldn't pass up the time I've lived in for any of those. I like my air conditioning and cars lol.